1. Field of the Invention
The present invention relates to a magneto-resistive element and a method for manufacturing the same, particularly to the structure of a TMR element.
2. Description of the Related Art
Conventionally, a GMR (Giant Magneto-Resistive) element using a spin valve (SV) film has widely been used as a magneto-resistive element for a hard disk drive, and, in recent years, a TMR (Tunnel Magneto-Resistive) element is attracting attention as a highly sensitive magneto-resistive element. The TMR element is formed by stacking a lower magnetic layer, a tunnel barrier layer, and an upper magnetic layer in this order. In one embodiment, the lower magnetic layer has a magnetization direction that is fixed relative to the external magnetic field (this layer is also called a pinned layer), and the upper layer has a magnetization direction that is variable in accordance with the external magnetic field (this layer is also called a free layer). When sense current is applied in the direction of the stacking of the layers, electrons flow from the upper layer to the lower layer (or in the inverse direction) beyond the energy barrier that is established by the tunnel barrier layer. This effect is called the tunneling effect. It is known that the resistance to the sense current changes in accordance with the relative angle between the magnetization direction of the upper layer and the magnetization direction of the lower layer. The change in the resistance to the sense current, or MR ratio, can be detected when the magnetization direction of the upper layer changes in accordance with the external magnetic field.
A TMR element detects the magnitude of the external magnetic field in this way, and reads magnetic data in a recording medium. Since the reproduction output of the magneto-resistive element depends on the MR ratio, and since a TMR element exhibits a significantly larger MR ratio than a conventional GMR element, a TMR element is advantageous for providing a high-power magneto-resistive element.
The tunnel barrier layer is usually made of non-magnetic and non-conductive materials, such as alumina, and it is known that a TMR element using magnesium oxide (MgO) as the tunnel barrier also exhibits a large MR ratio. In particular, it is known that MgO having a crystalline structure of {100} orientation exhibits a large MR ratio. Therefore, the MgO layer is typically annealed at a high temperature in order to obtain such a crystalline structure.
In K. Tsunekawa et al, “CoFeB/MgO/CoFeB/Magnetic Tunnel Junctions with High TMR and Low Junction Resistance”, Proceedings of INTERMAG 2005, Apr. 4, 2005, an exemplary TMR element that uses MgO as the tunnel barrier layer is disclosed. Similar information is found at http://www.jpo.go.jp/shiryou/index.htm (searched on 27 Jul., 2005) “Other information>standard technology>electricity>2004 fiscal year>MRAM and spin memory technology (Sections 1-2-2-3 to 1-2-2-6)” updated on Mar. 25, 2005 by Japan Patent Office, which discloses a layer arrangement in which the tunnel barrier made of MgO is sandwiched by CoFeB layers. Further, in INTERMAG 2005, held on 4 to 8 Apr., 2005, a study result was reported, in which the MR ratio significantly worsens for a tunnel barrier layer with a thickness that is less than a certain value. According to the report, the reason is that MgO is not crystallized in the initial state of forming MgO layer on the CoFeB layer, and that when the MgO layer reaches a certain thickness, a crystalline portion, which improves the MR ratio, is formed on the CoFeB layer.
As described above, it has been conventionally considered that a tunnel barrier layer in which a crystalline structure is dominant is effective for enhancing the MR ratio of a TMR element having a tunnel barrier layer that is made of MgO. However, the inventors discovered that the MR ratio is not always improved even if the crystalline structure of MgO is dominant. The inventors think that one of the reasons is in-plane stress in the tunnel barrier layer that is caused by other layers which are stacked together with the tunnel barrier layer.
FIG. 1 schematically shows the structure of a tunnel barrier layer. A TMR element, which is also used for a magnetic sensor and a magnetic memory element (MRAM, Magnetic Random Access Memory) in addition to the magneto-resistive element for the hard disk drive, as described above, generally has layers other than the tunnel barrier layer, the upper layer, and the lower layer regardless of the products to which a TMR element is applied. Taking a magnetic head as an example, a write head portion, not shown, is disposed on the upper layer (free layer), and an overcoat layer made of alumina or the like, not shown, is disposed on the write head portion. Various stresses are caused during the formation of these layers. In particular, formation of the overcoat layer generally causes compressive stress, which causes in-plane tensile stress in tunnel barrier layer 108. The tensile stress works to increase the distance between atoms in tunnel barrier layer 108, as indicated by the arrow in FIG. 1.
In general, it is thought that such stresses are caused at the interfaces between layers, and cooperate to generate a complicated stress state. In particular, the accumulation of such stresses and the resultant small cracks in tunnel barrier layer 108 may lead to the disadvantage that a leak path is generated for the sense current to intensively flow and thereby the MR ratio is reduced. Therefore, it is important to prevent in-plane stress in the tunnel barrier layer, as well as to crystallize the tunnel barrier layer, in order to improve the MR ratio.